It is well known that hemoglobin and oxyhemoglobin have different optical absorption spectra and that this difference in absorption spectra can be used as a basis for an optical oximeter. Most of the currently available oximeters using optical methods to determine blood oxygen saturation are based on transmission oximetry. These devices operate by transmitting light through an appendage such as a finger or an earlobe. By comparing the characteristics of the light transmitted into one side of the appendage with that detected on the opposite side, it is possible to compute oxygen concentrations. The main disadvantage of transmission oximetry is that it can only be used on portions of the body which are thin enough to allow passage of light.
Various methods and apparati for utilizing the optical properties of blood to measure blood oxygen saturation have been shown in the patent literature. Representative devices for utilizing the transmission method of oximetry have been disclosed in U.S. Pat. Nos. 4,586,513; 4,446,871; 4,407,290; 4,226,554; 4,167,331; and 3,998,550. Numerous other works have disclosed theoretical approaches for analyzing the behavior of light in blood and other materials. The following is a brief list of some of the most relevant of these references: "New Contributions to the Optics of Intensely Light-Scattering Materials, Part 1," by Paul Kubelka, Journal of the Optical Society of America, Volume 38, No. 5, May 1948; "Optical Transmission and Reflection by Blood," by R. J. Zdrojkowski and N. R. Pisharoty, IEEE Transactions on Biomedical Engineering, Vol. BME-17, No. 2, April 1970; "Optical Diffusion in Blood," by Curtis C. Johnson, IEEE Transactions on Biomedical Engineering, Vol. BME-17, No. 2, April 1970 and "Optical Scattering in Blood," by Narayanan R. Pisharoty, (Published Doctoral Dissertation), No. 7124861, University Microfilms, Ann Arbor, Mich. (1971).
There has been considerable interest in recent years in the development of an oximeter which is capable of using reflected light to measure blood oxygen saturation. A reflectance oximeter would be especially useful for measuring blood oxygen saturation in portions of the patient's body which are not well suited to transmission measurements. A theoretical discussion of a basis for the design of a reflectance oximeter is contained in "Theory and Development of a Transcutaneous Reflectance Oximeter System for Noninvasive Measurements of Arterial Oxygen Saturation," by Yitzhak Mendelson (Published Doctoral Dissertation), No. 8329355, University Microfilms, Ann Arbor, Mich. (1983). In addition, reflectance oximetry devices and techniques are shown generally in U.S. Pat. Nos. 4,447,150; 4,086,915; and 3,825,342. Recent work by the present inventor and other investigators has shown that it is possible to obtain accurate indications of blood oxygen saturation using reflectance oximetry techniques. One of the difficulties with previous reflectance oximeters, however, is the need to have prior information relating to the oxygen content of the patient's blood in order to construct an oxygen saturation reference curve for that particular patient. Such information is typically obtained by removing and analyzing a sample of the patient's blood or by measuring the patient's oxygen saturation with another monitoring device, such as a transmission oximeter.